Revert r331843 "[DebugInfo] Generate debug information for labels."

It broke the Chromium build (see reply on the review).

> Generate DILabel metadata and call llvm.dbg.label after label
> statement to associate the metadata with the label.
>
> Differential Revision: https://reviews.llvm.org/D45045
>
> Patch by Hsiangkai Wang.

This doesn't revert the change to backend-unsupported-error.ll
that seems to correspond to an llvm-side change.

llvm-svn: 331861